Most television systems utilize signals which have been generated by scanning a source image in a 2:1 interlaced format. Many display devices are available which are capable of reproducing such images directly from the interlaced signal. For instance, in a CRT monitor, the interlaced signal may be used directly to modulate the intensity of a beam as it is swept across the screen in an interlaced raster format. Such displays, however, may suffer from artifacts such as visible line structure, flicker and twitter which are related to the interlaced nature of the scanning. In particular, these undesirable artifacts tend to become more noticeable for screens with larger diagonal sizes. It is often desirable to convert signals from an interlaced scan format to a progressive scan format in order to reduce the artifacts associated with interlaced scanning. Furthermore, some display devices are inherently progressive in nature and therefore require conversion to a progressive format before display is possible.
A number of solutions to the problem of conversion from interlaced to progressive scan format have been proposed in the prior art. One such method involves the simple merging of two interlaced video fields to produce a progressively scanned video frame in which the even lines come from the even field and the odd lines come from the odd field. This technique works well for sequences which contain little or no motion but results in objectionable artifacts when motion is present due to the simultaneous display of video data which represents the image at different points in time.
Various forms of spatial and/or temporal interpolation have also been proposed. One such method involves spatial interpolation within a single interlaced field in order to produce a progressive frame. This approach does not suffer from the motion artifacts described above but, among other problems, suffers from a loss of vertical detail since each field contains only half of the spatial picture data. Alternatively, it is also possible to generate the missing lines by means of purely temporal interpolation. This approach yields maximum vertical detail for static images but results in serious blur when motion is present. Various attempts have also been made to combine spatial and temporal interpolation in order to reap the benefits of both approaches. As described in U.S. Pat. No. 4,789,893 (Weston), it is possible to generate the missing lines as a weighted average of neighboring lines from both the current and adjacent fields. The weightings applied to each of the neighboring lines are chosen such that low vertical frequency components are contributed mainly by the current field and higher vertical frequency components are contributed partly by the current field and partly by the adjacent fields. This approach has the benefit that vertical resolution is enhanced at low temporal frequencies by the contribution from the adjacent fields, however, for higher temporal frequencies the contribution from the adjacent fields actually reduces the vertical resolution. Although this last method does not suffer from motion blur artifacts, it has been found that further enhancement of the image is possible. According to the present invention, a method is provided whereby the apparent sharpness of moving detail may be enhanced in both spatial dimensions.
The following patents are relevant as prior art relative to the present invention:
U.S. Pat. Documents 4,789,893 - Weston Dec 6/88 Interpolating lines of video signals 3,920,889 - Connor Nov 18/75 Method and apparatus for crispening video signals by the use of temporal filters 4,792,854 - Glenn Dec 20/88 Apparatus for temporally processing a video signal 5,227,883 - Dischert et al. Jul 13/93 Method and apparatus for motion aperture correction